My phone is saying No SIM card installed, when I clearly have my SIM inserted?

Did you already try to remove it and put it back again?

When changing the SIM card, power off the device prior to opening the SIM tray. After you have changed the SIM card for your iPhone you may need to reconnect it to iTunes.

iPhone keeps a record of previously inserted SIM cards, but if the carrier name of your SIM does not show up in the status bar after you changed the SIM you will need to reconnect iPhone to iTunes. If you do not connect iPhone to iTunes after changing or reinserting the SIM, iPhone may not work as expected.

If iPhone does not work with the SIM card after connecting to iTunes, you might need to restore your iPhone. Consult the iPhone User's Guide for more information on iTunes and iPhone.

If your iPhone does not show up in iTunes (on the left hand side of the iTunes window under Devices), please see the following links for assistance:

• iPhone: Not recognized in iTunes for Mac OS X
• iPhone: Not recognized in iTunes for Windows

copied from http://support.apple.com/kb/HT2309 About swapping SIM cards

If this does not work, contact your provider, maybe your SIM is damaged.

it may be because the sim the sim must not have been cut properly......even i had that problem and i had to take a new sim.....(iphone 4)

I am having the same "No SIM card installed" error on an iPhone 4 that I have had for a week. It worked fine, but suddenly this morning it started saying "No SIM card installed". I used it fine on the way to work, but when I got to work, the problem came. I tried removing and reseating the SIM card, but it did not help.

I will be heading to the genius bar after my last class. :-(